Historically, one area of concern with painted exterior car and truck bodies is chip and scratch resistance. Utilizing an electro-deposition process, steel car bodies are primed with an organic coating prior to top coating. This primed steel is referred to as "E-coated steel". The coating may be any of a variety of polymeric materials known in the art which contain an ionic moiety which causes them to migrate to either the anode or cathode of a electrophoretic plating bath, the metal part to be plated serving as the cathode or anode according to the polymer. Electrodeposition is described, for example, in "Automotive Coatings" Bruce McBane, Federation of Societies for Coating Technology, September 1987. An advantage of E-coating of steel or other metals is that the build-up of polymers is self-limiting because the build-up of polymer on the metal renders the surface progressively less conductive. Accordingly, very uniform coatings are achieved. In most cases a second primer, referred to as a "primer-surfacer" is applied to the E-coated steel. The nominal film build on this primer-surfacer is 1.2 mils with a nominal bake of 25 minutes at 325 .degree. F. The topcoat is usually a 1-component, acid catalyzed polyol-melamine, a 2-component polyol-polyisocyanate system or a 1-component polyol-blocked polyisocyanate system. To a lesser extent, certain 2-component "non-isocyanate" coatings are utilized as exterior topcoats. The coating may be a single coat color, a dual coat (colorcoat/clearcoat) or a tri-coat (colorcoat/tinted clearcoat/clearcoat) system. All of the above systems have excellent durability as tested by UV light resistance and are categorized as hard, rigid, non-flexible coatings.
Flexible paints over flexible plastic substrates are utilized in areas of automobiles and trucks where impact and chip resistance are of concern. The areas include bumpers, grilles, and rocker panel areas. A flexible coating is defined herein as a coating when applied and baked on flexible substrate such as TPU (thermoplastic urethane) shows no cracking when subjected to a 1" mandrel bend at 75.degree. F., 0.degree. F., and -20.degree. F.
Previously, flexible coatings have not been deemed acceptable for application to rigid substrates such as E-coated steel because of certain deficiencies. The deficiencies include too soft (easily scratched and marred), not finessible (not repairable via a buffing and polishing procedure) and excessive dirt pick-up.